The present invention relates to lighting systems circuits. In particular, the present invention relates to circuitry to allow a lighting system to continue operation in case of failure of one or more of its light emitting elements.
The use of LED lighting technology is well known in the lighting art. However, rarely will one LED suffice to provide satisfactory illumination. As is common in the art, often multiple LEDs are electrically connected in series to provide illumination. However, the series arrangement is vulnerable to failure of one or more LED elements along the series string. If an LED fails so as to present a substantially higher resistance or open circuit, the remaining LED elements along the string will ordinarily cease to emit light.
Still, good reasons exist to place increased numbers of LEDs in series. For example, it may be advantageous to run a series circuit of many low current LEDs to reduce conductor cross-section. Additionally some power supplies may intrinsically produce high voltage output, requiring many LEDs in series to insure the individual voltage drop across each LED remains at a safe value. However, with increased numbers of LEDs in series, the possibility of at least one LED failure increases, thus posing a reliability risk.
One approach to minimize risk of light string failure due to just one failed LED or subset of failed LEDs along the string is to prevent LED damage through adding a protective element in parallel with each LED. For example, U.S. Pat. No. 6,054,716 issued to Sonobe et al on Apr. 25, 2000 discloses a parallel zener diode connected within an LED package to protect the LED from transient voltage.
Ideally, circuitry in parallel with a failed LED would allow current to flow through remaining LEDs despite the failed LED. The Sonobe patent discloses use of a zener diode to prevent LED failure but not to pass current around the LED in case of LED failure. Consequently in case of LED failure in Sonobe's circuit, there is no guarantee that any other LEDs connected in series with Sonobe's circuit would stay lit.
Accordingly, there remains a need for simple circuitry to economically sustain lighting system illumination in case of one or more LED failures along a series string.